Surface-coated cutting tool

ABSTRACT

A surface-coated cutting tool according to the present invention includes a coating. The coating includes an α-Al 2 O 3  layer. Each of the α-Al 2 O 3  layers on a side of a rake face and a side of a flank face shows (001) orientation. In the α-Al 2 O 3  layer on the rake face side, a length L R3  of a Σ3 crystal grain boundary exceeds 80% of a length L R3-29  of a Σ3-29 crystal grain boundary and is not lower than 10% and not higher than 50% of a total length L R  of all grain boundaries. In the α-Al 2 O 3  layer on the flank face side, a length L F3  of a Σ3 crystal grain boundary exceeds 80% of a length L F3-29  of a Σ3-29 crystal grain boundary and is not lower than 10% and not higher than 50% of a total length L F  of all grain boundaries. A ratio L R3 /L R3-29  is lower than a ratio L F3 /L F3-29 .

TECHNICAL FIELD

The present invention relates to a surface-coated cutting tool.

BACKGROUND ART

A surface-coated cutting tool having a coating formed on a base materialhas conventionally been used. For example, Japanese Patent Laying-OpenNo. 2006-198735 (PTD 1) discloses a surface-coated cutting tool having acoating including an α-Al₂O₃ layer in which a ratio of a Σ3 crystalgrain boundary in a Σ3-29 crystal grain boundary is 60 to 80%.

Japanese National Patent Publication No. 2014-526391 (PTD 2) discloses asurface-coated cutting tool having a coating including an α-Al₂O₃ layerin which a length of a Σ3 crystal grain boundary exceeds 80% of a lengthof a Σ3-29 crystal grain boundary.

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 2006-198735-   PTD 2: Japanese National Patent Publication No. 2014-526391

SUMMARY OF INVENTION Technical Problem

As a ratio of a Σ3 crystal grain boundary in grain boundaries includedin an α-Al₂O₃ layer is higher in a coating including the α-Al₂O₃ layercomposed of polycrystalline α-Al₂O₃, various characteristics representedby mechanical characteristics improve and hence resistance to wear andresistance to breakage are improved. It is thus expected that a cuttingtool is longer in life.

In recent working by cutting, however, a speed and efficiency havebecome high, load imposed on a cutting tool has increased, and life ofthe cutting tool has disadvantageously become short. Therefore, furtherimprovement in mechanical characteristics of a coating on the cuttingtool and longer life of the cutting tool have been demanded.

The present disclosure was made in view of such circumstances, and anobject thereof is to provide a surface-coated cutting tool achievingimproved mechanical characteristics of a coating and longer life of thecutting tool.

Solution to Problem

A surface-coated cutting tool according to one embodiment of the presentdisclosure has a rake face and a flank face, and includes a basematerial and a coating formed on the base material. The coating includesan α-Al₂O₃ layer, the α-Al₂O₃ layer contains a plurality of crystalgrains of α-Al₂O₃, and a grain boundary of the crystal grains includes aCSL grain boundary and a general grain boundary. The α-Al₂O₃ layer on arake face side shows (001) orientation, and in the α-Al₂O₃ layer on therake face side, a length L_(R3) of a Σ3 crystal grain boundary in theCSL grain boundary exceeds 80% of a length L_(R3-29) of a Σ3-29 crystalgrain boundary and is not lower than 10% and not higher than 50% of atotal length L_(R) of all grain boundaries which is a sum of lengthL_(R3-29) and a length L_(RG) of the general grain boundary. The α-Al₂O₃layer on a flank face side shows (001) orientation, and in the α-Al₂O₃layer on the flank face side, a length L_(F3) of a Σ3 crystal grainboundary in the CSL grain boundary exceeds 80% of a length L_(F3-29) ofa Σ3-29 crystal grain boundary and is not lower than 10% and not higherthan 50% of a total length L_(F) of all grain boundaries which is a sumof length L_(F3-29) and a length L_(FG) of the general grain boundary. Aratio L_(R3)/LR₃₋₂₉ of length L_(R3) to length L_(R3-29) is lower than aratio L_(F3)/L_(F3-29) of length L_(F3) to length L_(F3-29).

Advantageous Effects of Invention

According to the above, mechanical characteristics of a coating can beimproved and life of a cutting tool can further be longer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a surface-coated cutting tool accordingto one embodiment of the present disclosure.

FIG. 2 is a cross-sectional view along the line II-II in FIG. 1.

FIG. 3 is a cross-sectional view illustrating a surface-coated cuttingtool having a honed cutting edge ridgeline portion.

DESCRIPTION OF EMBODIMENTS Description of Embodiments of PresentInvention

Embodiments of the present disclosure will initially be listed anddescribed.

[1] A surface-coated cutting tool according to one embodiment of thepresent disclosure has a rake face and a flank face, and includes a basematerial and a coating formed on the base material. The coating includesan α-Al₂O₃ layer, the α-Al₂O₃ layer contains a plurality of crystalgrains of α-Al₂O₃, and a grain boundary of the crystal grains includes aCSL grain boundary and a general grain boundary. The α-Al₂O₃ layer on arake face side shows (001) orientation, and in the α-Al₂O₃ layer on therake face side, a length L_(R3) of a Σ3 crystal grain boundary in theCSL grain boundary exceeds 80% of a length L_(R3-29) of a Σ3-29 crystalgrain boundary and is not lower than 10% and not higher than 50% of atotal length L_(R) of all grain boundaries which is a sum of lengthL_(R3-29) and a length L_(RG) of the general grain boundary. The α-Al₂O₃layer on a flank face side shows (001) orientation, and in the α-Al₂O₃layer on the flank face side, a length L_(F3) of a Σ3 crystal grainboundary in the CSL grain boundary exceeds 80% of a length L_(F3-29) ofa Σ3-29 crystal grain boundary and is not lower than 10% and not higherthan 50% of a total length L_(F) of all grain boundaries which is a sumof length L_(F3-29) and a length L_(FG) of the general grain boundary. Aratio L_(R3)/LR₃₋₂₉ of length L_(R3) to length LR₃₋₂₉ is lower than aratio L_(F3)/L_(F3-29) of length L_(F3) to length L_(F3-29). Thissurface-coated cutting tool achieves improved mechanical characteristicsof a coating and longer life.

[2] In the surface-coated cutting tool, the CSL grain boundary isconstituted of the Σ3 crystal grain boundary, a Σ7 crystal grainboundary, a Σ11 crystal grain boundary, a Σ17 crystal grain boundary, aΣ19 crystal grain boundary, a Σ21 crystal grain boundary, a Σ23 crystalgrain boundary, and a Σ29 crystal grain boundary, the length L_(R3-29)is a total sum of lengths of the Σ3 crystal grain boundary, the Σ7crystal grain boundary, the Σ11 crystal grain boundary, the Σ17 crystalgrain boundary, the Σ19 crystal grain boundary, the Σ21 crystal grainboundary, the Σ23 crystal grain boundary, and the Σ29 crystal grainboundary in the α-Al₂O₃ layer on the rake face side, and lengthL_(F3-29) is a total sum of lengths of the Σ3 crystal grain boundary,the Σ7 crystal grain boundary, the Σ11 crystal grain boundary, the Σ17crystal grain boundary, the Σ19 crystal grain boundary, the Σ21 crystalgrain boundary, the Σ23 crystal grain boundary, and the Σ29 crystalgrain boundary in the α-Al₂O₃ layer on the flank face side.

[3] Preferably, the α-Al₂O₃ layer has a thickness from 2 to 20 μm. Thecharacteristics above are thus most effectively exhibited.

[4] Preferably, the α-Al₂O₃ layer has surface roughness Ra less than 0.2μm.

Thus, adhesive wear between a work material and a cutting edge of thetool is suppressed and consequently resistance to chipping of thecutting edge is improved.

[5] Preferably, the α-Al₂O₃ layer includes a point where an absolutevalue for compressive stress is maximal, in a region within 2 μm from asurface side of the coating, and the absolute value for compressivestress at the point is lower than 1 GPa. Thus, breakage of the cuttingedge of the tool due to mechanical and thermal fatigue which occursduring an intermittent cutting process is suppressed and consequentlyreliability of the cutting edge is improved.

[6] Preferably, the coating includes a TiC_(x)N_(y) layer between thebase material and the α-Al₂O₃ layer, and the TiC_(x)N_(y) layer containsTiC_(x)N_(y) satisfying atomic ratio relation of 0.6≦x/(x+y)≦0.8.Adhesion between the base material and the α-Al₂O₃ layer is thusimproved.

Details of Embodiments of Present Invention

A surface-coated cutting tool according to an embodiment of the presentinvention (hereinafter also denoted as the “present embodiment”) will bedescribed in further detail below with reference to FIGS. 1 to 3.

<Surface-Coated Cutting Tool>

Referring to FIG. 1, a surface-coated cutting tool 10 according to thepresent embodiment (hereinafter simply denoted as a “tool 10”) has arake face 1, a flank face 2, and a cutting edge ridgeline portion 3 atwhich rake face 1 and flank face 2 intersect with each other. Namely,rake face 1 and flank face 2 are surfaces connected to each other withcutting edge ridgeline portion 3 being interposed. Cutting edgeridgeline portion 3 implements a cutting edge tip end portion of tool10. Such a shape of tool 10 relies on a shape of a base material whichwill be described later.

Though FIG. 1 shows tool 10 representing a throwaway chip for turning,tool 10 is not limited thereto and the tool can suitably be used as acutting tool such as a drill, an end mill, a throwaway tip for a drill,a throwaway tip for an end mill, a throwaway tip for milling, a metalsaw, a gear cutting tool, a reamer, and a tap.

When tool 10 is implemented as a throwaway chip, tool 10 may or may nothave a chip breaker, and cutting edge ridgeline portion 3 may have asharp edge (a ridge at which a rake face and a flank face intersect witheach other) (see FIG. 1), may be honed (a sharp edge provided with R)(see FIG. 3), may have a negative land (beveled), and may be honed andhave a negative land.

Referring to FIG. 2, tool 10 has a base material 11 and a coating 12formed on base material 11. Though coating 12 preferably covers theentire surface of base material 11 in tool 10, a part of base material11 being not covered with coating 12 or a partially differentconstruction of coating 12 does not depart from the scope of the presentembodiment.

<Base Material>

Base material 11 according to the present embodiment has a rake face 11a, a flank face 11 b, and a cutting edge ridgeline portion 11 c at whichrake face 11 a and flank face 11 b intersect with each other. Rake face11 a, flank face 11 b, and cutting edge ridgeline portion 11 c implementrake face 1, flank face 2, and cutting edge ridgeline portion 3 of tool10, respectively.

For base material 11, any conventionally known base material of such akind can be employed. Such a base material is preferably exemplified bycemented carbide (for example, WC-based cemented carbide, which containsnot only WC but also Co, or to which a carbonitride of Ti, Ta, or Nb maybe added), cermet (mainly composed of TiC, TiN, or TiCN), high-speedsteel, ceramics (titanium carbide, silicon carbide, silicon nitride,aluminum nitride, or aluminum oxide), a cubic boron nitride sinteredobject, or a diamond sintered object. Among these various basematerials, in particular, WC-based cemented carbide or cermet (inparticular, TiCN-based cermet) is preferably selected. This is becausesuch base materials are particularly excellent in balance betweenhardness and strength at a high temperature and have characteristicsexcellent as a base material for the surface-coated cutting tool inapplications above.

<Coating>

Coating 12 according to the present embodiment may include other layersso long as it includes an α-Al₂O₃ layer. Examples of other layers caninclude a TiN layer, a TiCN layer, a TiBNO layer, a TiCNO layer, a TiB₂layer, a TiAlN layer, a TiAlCN layer, a TiAlON layer, and a TiAlONClayer. An order of layering is not particularly limited.

In the present embodiment, a chemical formula such as “TiN”, “TiCN,” or“TiC_(x)N_(y)” in which an atomic ratio is not particularly specified inthe present embodiment does not indicate that an atomic ratio of eachelement is limited only to “1” but encompasses all conventionally knownatomic ratios.

Such a coating 12 according to the present embodiment has a function toimprove various characteristics such as resistance to wear andresistance to chipping by covering base material 11.

Coating 12 has a thickness suitably of 3-30 μm (not smaller than 3 μmand not greater than 30 μm; a numerical range expressed with “-” in thepresent application refers to a range including upper limit and lowerlimit numeric values) and more preferably of 5-20 μm. When a thicknessis smaller than 3 μm, resistance to wear may be insufficient, and whenthe thickness exceeds 30 μm, peel-off or destruction of coating 12 mayoccur with high frequency during intermittent working, with applicationof a large stress between coating 12 and base material 11.

<α-Al₂O₃ Layer>

Coating 12 according to the present embodiment includes an α-Al₂O₃layer. Coating 12 can include one α-Al₂O₃ layer or two or more α-Al₂O₃layers.

The α-Al₂O₃ layer contains a plurality of crystal grains of α-Al₂O₃(aluminum oxide of which crystal structure is of an α type). Namely,this layer is composed of polycrystalline α-Al₂O₃. Normally, thesecrystal grains have a grain size approximately from 100 to 2000 nm. A“crystal grain boundary” is present between a plurality of crystalgrains of α-Al₂O₃.

The crystal grain boundary significantly affects characteristics of asubstance such as growth of crystal grains, creep characteristics,diffusion characteristics, electric characteristics, opticalcharacteristics, and mechanical characteristics. Importantcharacteristics to be taken into consideration include, for example, adensity of crystal grain boundaries in a substance, a chemicalcomposition of an interface, and a crystallographic texture, that is, acrystal grain boundary plane orientation and crystal misorientation. Inparticular, a coincidence site lattice (CSL) crystal grain boundaryplays a special role. The CSL crystal grain boundary (also simplyreferred to as a “CSL grain boundary”) is characterized by amultiplicity index E, and is defined as a ratio between a density ofsites of crystal lattices of two crystal grains in contact with eachother at the crystal grain boundary and a density of sites whichcoincide with each other when the crystal lattices are superimposed oneach other. It has generally been admitted that, in a simple structure,a crystal grain boundary having a low E value tends to have lowinterface energy and special characteristics. Therefore, control of aratio of a special crystal grain boundary and a crystal misorientationdistribution estimated from a CSL model is considered as important forcharacteristics of a ceramic coating and a method of improving thosecharacteristics.

A technique based on a scanning electron microscope (SEM) known aselectron beam backscattered diffraction (EBSD) has recently appeared andhas been used for studies of a crystal grain boundary in a ceramicsubstance. The EBSD technique is based on automatic analysis of aKikuchi diffraction pattern generated by backscattered electrons.

A crystallographic orientation of each crystal grain of a substance ofinterest is determined after indexing of a corresponding diffractionpattern. A texture is analyzed and a grain boundary characterdistribution (GBCD) is determined relatively easily with EBSD, with theuse of commercially available software. Crystal grain boundarymisorientation of a sample population having a large interface can bedetermined by applying EBSD to the interfaces. Misorientationdistribution is normally associated with a condition for treatment of asubstance. Crystal grain boundary misorientation can be obtained basedon a common orientation parameter such as an Euler angle, an angle/axispair, or a Rodrigues' vector. The CSL model is widely used as a tool fordetermining characteristics.

The crystal grain boundary in the α-Al₂O₃ layer according to the presentembodiment includes the CSL grain boundary and the general grainboundary described above. The CSL grain boundary is constituted of theΣ3 crystal grain boundary, the Σ7 crystal grain boundary, the Σ11crystal grain boundary, the Σ17 crystal grain boundary, the Σ19 crystalgrain boundary, the Σ21 crystal grain boundary, the Σ23 crystal grainboundary, and the Σ29 crystal grain boundary. Even when any one or morecrystal grain boundaries other than the Σ3 crystal grain boundary arenot observed in observation with EBSD, such a case does not depart fromthe scope of the present embodiment so long as an effect of the presentembodiment is exhibited. The general grain boundary refers to crystalgrain boundaries other than the CSL crystal grain boundary. Therefore,the general grain boundary refers to a remainder resulting fromexclusion of the CSL grain boundary from all grain boundaries of crystalgrains of α-Al₂O₃ in observation with EBSD.

The α-Al₂O₃ layer according to the present embodiment satisfies (1) to(4) below.

(1) Each of the α-Al₂O₃ layers on a rake face side and on a flank faceside shows (001) orientation.

(2) In the α-Al₂O₃ layer on the rake face side, length L_(R3) of the Σ3crystal grain boundary exceeds 80% of length L_(R3-29) of the Σ3-29crystal grain boundary and is not lower than 10% and not higher than 50%of total length L_(R) of all grain boundaries which is a sum ofL_(R3-29) and length L_(RG) of the general grain boundary.

(3) In the α-Al₂O₃ layer on the flank face side, length L_(F3) of the Σ3crystal grain boundary exceeds 80% of length L_(F3-29) of the Σ3-29crystal grain boundary and is not lower than 10% and not higher than 50%of total length L_(F) of all grain boundaries which is a sum ofL_(F3-29) and length L_(FG) of the general grain boundary.

(4) Ratio L_(R3)/L_(R3-29) of length L_(R3) to length L_(R3-29) is lowerthan ratio L_(F3)/L_(F3-29) of length L_(F3) to length L_(F3-29).

(1) above will be described. The α-Al₂O₃ layer on a side of each surface“showing (001) orientation” herein refers to such a condition that aratio (an area ratio) of crystal grains (α-Al₂O₃) of which normaldirection to a (001) plane is within ±20° with respect to a normaldirection to a surface of the α-Al₂O₃ layer (a surface located on asurface side of the coating) is not lower than 50% in the α-Al₂O₃ layer.Specifically, it refers to such a condition that, when a verticalcross-section of the α-Al₂O₃ layer (a cross-section in parallel to thenormal direction to the surface of the α-Al₂O₃ layer) on each of therake face side and the flank face side is observed with an SEM with EBSDand a result thereof is subjected to image processing with colormapping, an area ratio of the above-described crystal grains in theα-Al₂O₃ layer in each observation image subjected to image processing isnot lower than 50%.

All areas of the α-Al₂O₃ layer in a direction of thickness are subjectedto color mapping of the α-Al₂O₃ layer. Namely, all regions from thesurface located on a cover surface side of the α-Al₂O₃ layer to thesurface located on a side of the base material of the α-Al₂O₃ layer issubjected to color mapping. All areas in the direction of thickness ofthe α-Al₂O₃ layer having a thickness of the order of micrometer can beobserved in one observation image. On the other hand, any area in anin-plane direction (a direction orthogonal to the direction ofthickness) of the α-Al₂O₃ layer should only be subjected to colormapping.

In general, as the α-Al₂O₃ layer is highly oriented in line with the(001) plane, a hardness of the α-Al₂O₃ layer is higher. Therefore,according to the α-Al₂O₃ layer in the present embodiment which satisfies(1), occurrence of a crack due to impact applied during working can besuppressed, toughness of the cutting tool can significantly be improved,and hence high resistance to wear can be achieved.

The “surface side of the coating” means a side opposite to the side ofthe base material in the direction of thickness of the α-Al₂O₃ layer,and when no other layer is formed on the α-Al₂O₃ layer, it means thesurface of the α-Al₂O₃ layer.

(2) and (3) above will be described. The Σ3 crystal grain boundary isconsidered as lowest in grain boundary energy among CSL crystal grainboundaries of α-Al₂O₃, and hence it is considered that mechanicalcharacteristics (in particular, resistance to plastic deformation) canbe enhanced by increasing a ratio thereof in all CSL crystal grainboundaries. Therefore, in the present embodiment, the all CSL crystalgrain boundaries are denoted as the Σ3-29 crystal grain boundary, andlength L_(R3) of the Σ3 crystal grain boundary in the α-Al₂O₃ layer onthe rake face side is defined as exceeding 80% of length L_(R3-29) ofthe Σ3-29 crystal grain boundary in the α-Al₂O₃ layer on the rake faceside and length L_(F3) of the Σ3 crystal grain boundary in the α-Al₂O₃layer on the flank face side is defined as exceeding 80% of lengthL_(F3-29) of the Σ3-29 crystal grain boundary in the α-Al₂O₃ layer onthe flank face side.

Length L_(R3) refers to a total length of the Σ3 crystal grain boundaryin a field of view observed in observation with an SEM with EBSD of theα-Al₂O₃ layer on the rake face side, and length L_(R3-29) refers to thetotal sum of lengths of the Σ3 crystal grain boundary, the Σ7 crystalgrain boundary, the Σ11 crystal grain boundary, the Σ17 crystal grainboundary, the Σ19 crystal grain boundary, the Σ21 crystal grainboundary, the Σ23 crystal grain boundary, and the Σ29 crystal grainboundary in a field of view observed in observation with an SEM withEBSD of the α-Al₂O₃ layer on the rake face side. Similarly, lengthL_(F3) refers to a total length of the Σ3 crystal grain boundary in afield of view observed in observation with an SEM with EBSD of theα-Al₂O₃ layer on the flank face side, and length L_(F3-29) refers to thetotal sum of lengths of the Σ3 crystal grain boundary, the Σ7 crystalgrain boundary, the Σ11 crystal grain boundary, the Σ17 crystal grainboundary, the Σ19 crystal grain boundary, the Σ21 crystal grainboundary, the Σ23 crystal grain boundary, and the Σ29 crystal grainboundary in a field of view observed in observation with an SEM withEBSD of the α-Al₂O₃ layer on the flank face side.

Length L_(R3) is more preferably 83% or higher and further preferably85% or higher of length L_(R3-29). This is also applicable to lengthL_(F3), and length L_(F3) is more preferably 83% or higher and furtherpreferably 85% or higher of length L_(F3-29). A numeric value is thuspreferably as high as possible, and an upper limit thereof does not haveto be defined. From a point of view of a thin film beingpolycrystalline, however, the upper limit is 99% or lower.

Since the Σ3 crystal grain boundary has high conformity as is clear alsofrom the fact that it is low in grain boundary energy, two crystalgrains of which grain boundary is defined by the Σ3 crystal grainboundary exhibit a behavior similar to a behavior of a single crystal ortwin crystals and tend to be coarser. As crystal grains are coarser,characteristics of a coating such as resistance to chipping lower andhence coarsening should be suppressed. Therefore, in the presentembodiment, the suppression effect above is ensured by definingrespective lengths L_(R3) and L_(F3) of the Σ3 crystal grain boundary tobe not lower than 10% and not higher than 50% of total lengths L_(R) andL_(F) of all grain boundaries in the α-Al₂O₃ layer located on the sidesof the rake face and the flank face.

When respective lengths L_(R3) and L_(F3) of the Σ3 crystal grainboundary exceed 50% of total lengths L_(R) and L_(F) of all grainboundaries on the side of respective surfaces, crystal grainsunfavorably become coarser, and when the lengths are lower than 10%,excellent mechanical characteristics cannot be obtained. A morepreferred range of lengths L_(R3) and L_(F3) of the Σ3 crystal grainboundary is from 20 to 45% and a further preferred range is from 30 to40%.

All grain boundaries refer to the CSL crystal grain boundary and thegeneral crystal grain boundary other than the CSL crystal grain boundaryas being added. Therefore, “total length L_(R) of all grain boundaries”on the side of the rake face can be expressed as the “sum of lengthL_(R3-29) of the Σ3-29 crystal grain boundary and length L_(RG) of thegeneral grain boundary” and “total length L_(F) of all grain boundaries”on the side of the flank face can be expressed as the “sum of lengthL_(F3-29) of the Σ3-29 crystal grain boundary and length L_(FG) of thegeneral grain boundary.”

(4) above will be described. The α-Al₂O₃ layer satisfying (4) isrelatively high in ratio occupied by the Σ3 crystal grain boundary inthe CSL grain boundary in the α-Al₂O₃ layer on the side of the rake faceand relatively low in ratio of the Σ3 crystal grain boundary in the CSLgrain boundary in the α-Al₂O₃ layer on the side of the flank face.Therefore, the α-Al₂O₃ layer satisfying (2) and (3) and furthersatisfying (4) is particularly excellent in resistance to plasticdeformation on the side of the flank face and can sufficiently suppresslowering in characteristics of the coating such as resistance tochipping attributed to too much Σ3 crystal grain boundary on the side ofthe rake face.

In a cutting process under a high-speed condition and a low-feedcondition (hereinafter also denoted as a “high-speed and low-feedcutting process”), thermal load on the side of the flank face is highand wear of the flank face tends to increase. The α-Al₂O₃ layersatisfying (2) to (4) can withstand load applied to the side of theflank face, and hence long life can be maintained also in the high-speedand low-feed cutting process under severe cutting conditions inparticular on the side of the flank face.

A difference {(L_(R3)/L_(R3-29))−(L_(F3)/L_(F3-29))} between ratioL_(R3)/L_(R3-29) and ratio L_(F3)/L_(F3-29) is preferably from −1 to−10. In this case, balance between improvement in resistance to chippingon the side of the rake face and improvement in resistance to plasticdeformation on the side of the flank face is excellent. The differenceis more preferably from −4 to −9. In order to satisfy such a difference,length L_(R3) of the Σ3 crystal grain boundary is preferably from 80 to95%, more preferably from 83 to 95%, and particularly preferably from 80to 90%, of length L_(R3-29) of the Σ3-29 crystal grain boundary. LengthL_(F3) of the Σ3 crystal grain boundary is preferably from 90 to 99% andmore preferably from 91 to 99% of length L_(F3-29) of the Σ3-29 crystalgrain boundary.

In the present embodiment, whether or not the α-Al₂O₃ layer satisfies(1) can be determined as follows.

Initially, an α-Al₂O₃ layer is formed on the base material based on amanufacturing method which will be described later. Then, the formedα-Al₂O₃ layer (including the base material) on the side of the rake faceis cut to obtain a cross-section perpendicular to the α-Al₂O₃ layer(that is, cut to expose a cut surface obtained by cutting the α-Al₂O₃layer along a plane including a normal line to the surface of theα-Al₂O₃ layer). Thereafter, the cut surface is polished with waterresistant sandpaper (which contains an SiC grain abrasive as anabrasive).

The α-Al₂O₃ layer is cut, for example, in such a manner that the surfaceof the α-Al₂O₃ layer (when another layer is formed on the α-Al₂O₃ layer,a surface of the coating) is fixed with the use of wax or the like asbeing in intimate contact to a sufficiently large flat plate forholding, and thereafter the α-Al₂O₃ layer is cut in a directionperpendicular to the flat plate with a cutter with a rotary blade (cutsuch that the rotary blade and the flat plate are as perpendicular aspossible to each other). Any portion of the α-Al₂O₃ layer can be cut solong as the α-Al₂O₃ layer is cut in such a perpendicular direction.

Polishing is performed successively with water resistant sandpaper #400,#800, and #1500 (the number (#) of the water resistant sandpaper means adifference in grain size of the abrasive, and a greater number indicatesa smaller grain size of the abrasive).

In succession, the polished surface is further smoothened through ionmilling treatment with the use of Ar ions. Conditions for ion millingtreatment are as follows.

Acceleration voltage: 6 kV

Irradiation angle: 0° from a direction of normal to the surface of theα-Al₂O₃ layer (that is, a linear direction in parallel to a direction ofthickness of the α-Al₂O₃ layer at the cut surface)

Irradiation time period: 6 hours

A position of cutting is set to avoid at least a portion in the vicinityof cutting edge ridgeline portion 3. Specifically, referring to FIG. 3,a region connecting an outer edge A₁ of a region where a surface incontact with rake face 1 (shown with a dotted line extending laterallyin FIG. 3) and rake face 1 are in contact with each other and an outeredge A₂ of a region where a surface in contact with flank face 2 (shownwith a dotted line extending vertically in FIG. 3) and flank face 2 arein contact with each other is regarded as cutting edge ridgeline portion3, and a position distant by 0.2 mm or greater from outer edge A₁ whichis an end portion of cutting edge ridgeline portion 3 (a positiondistant from outer edge A₁ to the left in FIG. 3) is applied as aposition of a cross-section. Similarly also in a cross-section on theside of flank face 2 of the α-Al₂O₃ layer, a position distant by 0.2 mmor greater from outer edge A₂ which is an end portion of cutting edgeridgeline portion 3 (a position distant from outer edge A₂ in a downwarddirection in FIG. 3) is applied as a position of a cross-section.

The smoothened polished surface is observed with an SEM with EBSD. ZeissSupra 35 VP (manufactured by CARL ZEISS) including an HKL NL02 EBSDdetector is employed as the SEM. EBSD data is successively collected byindividually positioning focused electron beams onto each pixel.

A normal line to a sample surface (a smoothened cross-section of theα-Al₂O₃ layer) is inclined by 70° with respect to incident beams, andanalysis is conducted at 15 kV. In order to avoid a charging effect, apressure of 10 Pa is applied. A high current mode is set in conformitywith a diameter of an opening of 60 μm or 120 μm. Data is collectedstepwise at 0.1 μm/step, for 500×300 points corresponding to a planeregion of 50×30 m on the polished surface.

Then, with commercially available software (a trademark: “orientationImaging microscopy Ver 6.2” manufactured by EDAX Inc.), an angle formedbetween a direction of normal to the (001) plane of each measured pixeland a direction of normal to the surface of the α-Al₂O₃ layer (thesurface located on the surface side of the coating) (that is, the lineardirection in parallel to the direction of thickness of the α-Al₂O₃ layerat the cut surface) is calculated, and a color map in which a pixelhaving the angle within ±20° is selected is created.

Specifically, with the technique according to “Crystal Direction MAP”included in the software, a color map of Tolerance of 20° (a differencein direction being within ±20°) between the direction of normal to thesurface of the α-Al₂O₃ layer and the direction of normal to the (001)plane of each measured pixel is created. Then, an area ratio of thepixel is calculated based on this color map and the area ratio being 50%or higher is defined as “the α-Al₂O₃ layer on the side of the rake faceshowing (001) orientation.”

Similarly, whether or not the α-Al₂O₃ layer on the side of the flankface shows (001) orientation is determined. When it is confirmed thatthe α-Al₂O₃ layer on the side of any surface shows (001) orientation,that is, a ratio of crystal grains (α-Al₂O₃) of which direction ofnormal to the (001) plane is within ±20° with respect to the directionof normal to the surface (the surface located on the surface side of thecoating) of the α-Al₂O₃ layer is not lower than 50%, the α-Al₂O₃ layersatisfies (1).

In the present embodiment, whether or not each of the α-Al₂O₃ layer onthe side of the rake face and the α-Al₂O₃ layer on the side of the flankface satisfies (2) and (3) and whether or not it further satisfies (4)can be determined as follows.

Initially, the α-Al₂O₃ layer on the side of the rake face is cut toobtain a cross-section perpendicular to the α-Al₂O₃ layer similarly tothe above, and thereafter polishing and smoothing treatment aresimilarly carried out. Then, the cut surface thus treated is observedwith an SEM with EBSD as above.

A normal line to a sample surface (a smoothened cross-section of theα-Al₂O₃ layer) is inclined by 70° with respect to incident beams, andanalysis is conducted at 15 kV. In order to avoid a charging effect, apressure of 10 Pa is applied. A high current mode is set in conformitywith a diameter of an opening of 60 μm or 120 μm. Data is collectedstepwise at 0.1 μm/step, for 500×300 points corresponding to a planeregion of 50×30 m on the polished surface.

Data is processed with and without noise filtering. Noise filtering andcrystal grain boundary character distribution are determined by usingcommercially available software (a trademark: “orientation Imagingmicroscopy Ver 6.2” manufactured by EDAX Inc.). The crystal grainboundary character distribution is analyzed based on data available fromGrimmer (H. Grimmer, R. Bonnet, Philosophical Magazine A 61 (1990),493-509). With Brandon criterion (ΔΘ<Θ₀ (Σ)^(−0.5), where Θ₀=15°), atolerance of an experimental value from a theoretical value is takeninto account (D. Brandon Acta metall. 14 (1966), 1479-1484). Specialcrystal grain boundaries corresponding to any Σ value are counted, andthe count is expressed as a ratio to all crystal grain boundaries.

As set forth above, length L_(R3) of the Σ3 crystal grain boundary,length L_(R3-29) of the Σ3-29 crystal grain boundary, and total lengthL_(R) of all grain boundaries in the α-Al₂O₃ layer on the side of therake face can be found. Similarly, length L_(F3) of the Σ3 crystal grainboundary, length L_(F3-29) of the Σ3-29 crystal grain boundary, andtotal length L_(F) of all grain boundaries in the α-Al₂O₃ layer on theside of the flank face can be found. Based on the found values, whetheror not the α-Al₂O₃ layer on the side of the rake face and the α-Al₂O₃layer on the side of the flank face satisfy (2) and (3) can bedetermined and whether or not the α-Al₂O₃ layers further satisfy (4) canbe determined.

<Thickness of α-Al₂O₃ Layer>

The α-Al₂O₃ layer preferably has a thickness from 2 to 20 μm. Theexcellent effect as above can thus be exhibited. The thickness is morepreferably from 2 to 15 μm and further preferably from 2 to 10 μm.

When the thickness is smaller than 2 μm, the excellent effect as abovemay not sufficiently be exhibited. When the thickness exceeds 20 μm,interface stress attributed to a difference in coefficient of linearexpansion between the α-Al₂O₃ layer and another layer such as anunderlying layer increases and crystal grains of α-Al₂O₃ may come off.Such a thickness can be determined by observing a vertical cross-sectionof the base material and the coating with a scanning electron microscope(SEM).

<Surface Roughness of α-Al₂O₃ Layer>

The α-Al₂O₃ layer has surface roughness Ra preferably less than 0.2 μm.Thus, not only a coefficient of friction between chips and a cuttingedge of a tool lowers and resistance to chipping improves but alsostable capability to discharge chips can be exhibited. Surface roughnessRa is more preferably less than 0.15 μm and further preferably less than0.10 μm. Surface roughness Ra is thus preferably as low as possible, anda lower limit thereof does not have to be defined. From a point of viewof the fact that a coating is affected by a surface texture of the basematerial, however, the lower limit is 0.05 μm or greater.

In the present application, surface roughness Ra means arithmetical meanroughness Ra defined under JIS B 0601 (2001).

<Compressive Stress of α-Al₂O₃ Layer>

The α-Al₂O₃ layer preferably includes a point where an absolute valuefor compressive stress is maximal, in a region within 2 μm from asurface side of the coating, and the absolute value for compressivestress at the point is lower than 1 GPa. Thus, sudden breakage of acutting edge due to mechanical and thermal fatigue of the cutting edgeof the tool which occurs during an intermittent cutting process issuppressed and a manpower saving/energy saving effect can be exhibited.The absolute value is more preferably lower than 0.9 GPa and furtherpreferably lower than 0.8 GPa. Though the lower limit of the absolutevalue is not particularly limited, from a point of view of balancebetween resistance to wear and resistance to breakage, the lower limitis not lower than 0.2 GPa.

Compressive stress in the present embodiment can be measured with theconventionally known sin²ψ method and constant penetration depth methodwhich use X-rays.

<TiC_(x)N_(y) Layer>

The coating according to the present embodiment can include aTiC_(x)N_(y) layer between the base material and the α-Al₂O₃ layer. ThisTiC_(x)N_(y) layer preferably contains TiC_(x)N_(y) satisfying atomicratio relation of 0.6≦x/(x+y)≦0.8. Adhesion between the base materialand the α-Al₂O₃ layer is thus improved.

The atomic ratio is more preferably 0.65≦x/(x+y)≦0.75 and furtherpreferably 0.67≦x/(x+y)≦0.72. When x/(x+y) is smaller than 0.6,resistance to wear may be insufficient, and when it exceeds 0.8,resistance to chipping may be insufficient.

<Manufacturing Method>

The surface-coated cutting tool according to the present embodiment canbe manufactured by forming a coating on a base material through chemicalvapor deposition (CVD). When a layer other than the α-Al₂O₃ layer isformed in the coating, such a layer can be formed under conventionallyknown conditions with the use of a chemical vapor deposition apparatus.The α-Al₂O₃ layer can be formed as below.

AlCl₃, HCl, CO₂, H₂S, O₂, and H₂ are employed as source gases. Amountsof blend of AlCl₃, HCl, CO₂, H₂S, and O₂ are set to 3 to 5 volume %, 4to 6 volume %, 0.5 to 2 volume %, 1 to 5 volume %, and 0.0001 to 0.01volume %, respectively, and H₂ is adopted as the remainder. Volumeratios of 0.1≦CO₂/H₂S≦1, 0.1≦CO₂/AlCl₃≦1, and 0.5≦AlCl₃/HCl≦1 areadopted.

Though the source gas is blown to the base material arranged in areaction vessel in the chemical vapor deposition apparatus, a directionof injection of the source gas is adjusted such that the flank face ofthe base material is substantially perpendicular to the direction ofinjection of the source gas and the rake face of the base material issubstantially in parallel to the direction of injection of the sourcegas. Various conditions for chemical vapor deposition include atemperature from 950 to 1050° C., a pressure from 1 to 5 kPa, and a gasflow rate (a total amount of gases) from 50 to 100 L/min. A speed ofintroduction of the source gas into the reaction vessel is set to 1.7 to3.5 m/sec.

After the α-Al₂O₃ layer is once formed through chemical vapor depositionunder the conditions described above, annealing is performed. Conditionsfor annealing include a temperature from 1050 to 1080° C., a pressurefrom 50 to 100 kPa, and a time period from 120 to 300 minutes. Anatmosphere for this annealing is obtained by feeding H₂ and argon (Ar)each at a flow rate of 20 to 30 L/min.

The α-Al₂O₃ layer according to the present embodiment having a desiredthickness can thus be formed. In particular, by setting an amount ofblend of 02 in the source gas to the range above and adjusting adirection of injection of the source gas such that the flank face of thebase material is substantially perpendicular to the direction ofinjection of the source gas and the rake face of the base material issubstantially in parallel to the direction of injection of the sourcegas, the α-Al₂O₃ layer satisfying (2) to (4) can be formed. The reasonis estimated as follows.

O₂ is more reactive than other gases such as CO₂, and it has a functionto increase the number of nuclei of α-Al₂O₃ or to increase a rate offilm formation. O₂ can also have a function to lower a ratio of the Σ3crystal grain boundary in the CSL crystal grain boundary. This isbecause generation of the Σ3 crystal grain boundary which is the crystalgrain boundary having high conformity is less likely when a rate of filmformation is too high.

On the side of the rake face substantially in parallel to the directionof injection of the source gas, a flux density of a source gas tends torelatively be high, and on the side of the flank face substantiallyperpendicular to the direction of injection of the source gas, a fluxdensity of a source gas tends to be relatively low. Namely, a residencetime of a source gas tends to be short on the side of the rake face, anda residence time of a source gas tends to be long on the side of theflank face. In other words, the side of the rake face tends to besupplied with a source gas more frequently than the side of the flankface.

Therefore, apparently, adsorption and diffusion of O₂ on the side of therake face is more frequent than adsorption and diffusion of O₂ on theside of the flank face, which causes decrease in the Σ3 crystal grainboundary attributed to the function of O₂ on the side of the rake face,and consequently, a ratio of the Σ3 crystal grain boundary on the sideof the flank face is higher than on the side of the rake face.

Annealing as above after film formation can prevent an impurity such assulfur from remaining in the α-Al₂O₃ layer. Therefore, the method ofmanufacturing the α-Al₂O₃ layer according to the present embodiment isparticularly excellent.

EXAMPLES

Though the present invention will be described in further detail belowwith reference to Examples, the present invention is not limitedthereto.

<Preparation of Base Material>

Two types of base materials of a base material P and a base material Kshown in Table 1 below were prepared. Specifically, a base material madeof cemented carbide and having a shape of CNMG120408NUX (manufactured bySumitomo Electric Industries, Ltd.) was obtained by uniformly mixingsource material powders as formulated as shown in Table 1, forming thepowders into a prescribed shape by applying a pressure, and sinteringthe formed powders for 1 to 2 hours at 1300 to 1500° C.

TABLE 1 Formulated Composition (Mass %) Co Ni TiN TiC Mo₂C TaC WC P 5.0— 0.5 0.5 — 2.0 Remainder K 4.0 6.0 15.0 Remainder 10.0 — Remainder

<Formation of Coating>

A coating was formed on a surface of each base material obtained asabove. Specifically, a coating was formed on the base material throughchemical vapor deposition, with the base material being set in achemical vapor deposition apparatus. The base material was arranged inthe reaction vessel such that the rake face was substantially inparallel to a direction of injection of a gas and the flank face wassubstantially orthogonal to the direction of injection of the gas.

Conditions for forming the coating are as shown in Tables 2 and 3 below.Table 2 shows conditions for forming each layer other than the α-Al₂O₃layer, and Table 3 shows conditions for forming the α-Al₂O₃ layer. TiBNOand TiCNO in Table 2 represent an intermediate layer in Table 5 whichwill be described later, and other components correspond to layersexcept for the α-Al₂O₃ layer in Table 5. The TiC_(x)N_(y) layer iscomposed of TiC_(x)N_(y) in which an atomic ratio x/(x+y) is set to 0.7.

As shown in Table 3, there are 10 conditions of A to G and X to Z forforming the α-Al₂O₃ layer, and A to G correspond to the conditions inExamples and X to Z correspond to the conditions in Comparative Examples(conventional art). In formation of the α-Al₂O₃ layer, a speed ofintroduction of a source gas was set to 2 m/sec., and a gas pipe forinjection of the source gas was rotated at 2 rpm while the base materialwas fixed. Only the α-Al₂O₃ layer in Examples formed under conditions Ato G was annealed under conditions of 1050° C., 50 kPa, a flow rate ofH₂ being set to 20 L/min., and a flow rate of Ar being set to 30 L/min.for an annealing time period shown in Table 3.

For example, formation condition A indicates that the α-Al₂O₃ layer isformed by supplying a source gas composed of 3.2 volume % of AlCl₃, 4.0volume % of HCl, 1.0 volume % of CO₂, 2 volume % of H₂S, 0.003 volume %of O₂, and remainder H₂ to the chemical vapor deposition apparatus,performing chemical vapor deposition under conditions of a pressure of3.5 kPa, a temperature of 1000° C., and a flow rate (a total amount ofgases) of 70 L/min., and thereafter performing annealing for 180 minutesunder the conditions above.

Each layer other than the α-Al₂O₃ layer shown in Table 2 was formedsimilarly through chemical vapor deposition, except for not performingannealing. The “remainder” in Table 2 indicates that H₂ occupies theremainder of the source gases. The “total amount of gases” indicates atotal volume flow rate introduced into the chemical vapor depositionapparatus per unit time, with a gas in a standard condition (0° C. and 1atmospheric pressure) being defined as the ideal gas (also applicable tothe α-Al₂O₃ layer in Table 3). A thickness of each layer was adjusted byadjusting as appropriate a time period for film formation (a rate offilm formation of each layer was approximately from 0.5 to 2.0 μm/hour).

TABLE 2 Conditions for Film Formation Pressure Temperature Total Amountof Composition of Source Gas (Volume %) (kPa) (° C.) Gases (L/min) TiNTiCl₄ = 2.0%, N₂ = 39.7%, H₂ = Remainder 50 880 52.5 (Underlying Layer)TiN TiCl₄ = 0.5%, N₂ = 41.2%, H₂ = Remainder 80 980 63.7 (OutermostLayer) TiC_(x)N_(y) TiCl₄ = 2.0%, CH₃CN = 0.7%, 9 800 84.5 C₂H₄ = 1.5%,H₂ = Remainder TiBNO TiCl₄ = 36.7%, BCl₃ = 0.1%, CO = 1.6%, 5 970 54.7CO₂ = 1.7%, N₂ = 61.7%, H₂ = Remainder TiCNO TiCl₄ = 2.1%, CO = 3.2%,CH₄ = 2.8%, 15 1000 60.8 N₂ = 23.7%, H₂ = Remainder

TABLE 3 Composition of Source Gas Volume Ratio Conditions for FilmFormation Annealing (Volume %) CO₂/ CO₂/ AlCl₃/ Pressure TemperatureTotal Amount of Time AlCl₃ HCl CO₂ H₂S O₂ H₂ H₂S AlCl₃ HCl (kPa) (° C.)Gases (L/min) (min) Example A 3.2 4.0 1.0 2 0.0030 Remainder 0.50 0.30.8 3.5 1000 70 180 B 3.5 5.0 0.8 1.8 0.0060 Remainder 0.44 0.2 0.7 3.51000 70 180 C 3.0 6.0 0.5 1.6 0.0800 Remainder 0.31 0.2 0.5 3.0 1010 60120 D 4.4 5.0 1.0 1.1 0.0005 Remainder 0.91 0.2 0.9 4.0 980 75 240 E 5.26.0 1.6 2 0.0075 Remainder 0.80 0.3 0.9 3.5 1000 70 180 F 3.8 4.5 2.0 30.0009 Remainder 0.67 0.5 0.8 4.0 980 80 300 G 5.0 5.5 1.0 5 0.0100Remainder 0.20 0.2 0.9 3.0 1020 60 180 Comparative X 10 5.0 15 0.2 —Remainder 75 1.5 2.0 5.0 1050 50 — Example Y 2 2.0 3 0.5 — Remainder 61.5 1.0 8.0 1020 55 — Z 2 1.5 6 0.05 — Remainder 120 3.0 1.3 6.0 1020 50—

<Fabrication of Surface-Coated Cutting Tool>

Surface-coated cutting tools in Examples 1 to 15 and ComparativeExamples 1 to 6 shown in Tables 4 and 5 below were each fabricated byforming a coating on the base material under the conditions in Tables 2and 3.

In Tables 4 and 5, a composition and a thickness of each coating weredetermined with an SEM-EDX (scanning electron microscope-energydispersive X-ray spectrometry), and a length of the Σ3 crystal grainboundary, a length of the Σ3-29 crystal grain boundary, and a totallength of all grain boundaries of the α-Al₂O₃ layer were determined withthe method described above. A ratio (%) of crystal grains (α-Al₂O₃) ofwhich normal direction to the (001) plane is within ±20° with respect tothe normal direction to the surface of the α-Al₂O₃ layer (the surfacelocated on the surface side of the coating) in the α-Al₂O₃ layers on theside of the rake face and on the side of the flank face was determinedwith the method described above.

For example, referring to Table 4, the surface-coated cutting tool inExample 1 has a coating having a total thickness of 24.3 μm formed onthe base material, by adopting base material P shown in Table 1 as thebase material, forming a TiN layer having a thickness of 1.2 μm as anunderlying layer on the surface of base material P under the conditionsin Table 2, forming a TiC_(x)N_(y) layer having a thickness of 13.0 μmon the underlying layer under the conditions in Table 2, forming a TiBNOlayer having a thickness of 0.7 μm as an intermediate layer on theTiC_(x)N_(y) layer under the conditions in Table 2, fabricating theα-Al₂O₃ layer having a thickness of 8.6 μm on the intermediate layerunder the formation condition A in Table 3, and thereafter forming a TiNlayer having a thickness of 0.9 μm as an outermost layer under theconditions in Table 2. Blank fields in Table 4 indicate absence of acorresponding layer.

Referring to Table 5, in Example 1, in the α-Al₂O₃ layer on the side ofthe rake face, length L_(R3) of the Σ3 crystal grain boundary is 89% oflength L_(R3-29) of the Σ3-29 crystal grain boundary and 17% of totallength L_(R) of all grain boundaries. In the α-Al₂O₃ layer on the sideof the flank face, length L_(F3) of the Σ3 crystal grain boundary is 94%of length L_(F3-29) of the Σ3-29 crystal grain boundary and 20% of totallength L_(F) of all grain boundaries. A value calculated by subtractinga ratio (L_(F3)/L_(F3-29)) (%) from a ratio (L_(R3)/L_(R3-29)) (%) isshown in a field of “(L_(R3)/L_(R3-29))−(L_(F3)/L_(F3-29))”. Since thisvalue is “−”, it can be seen that ratio L_(R3)/L_(R3-29) is lower thanratio L_(F3)/L_(F3-29). This ca-Al₂O₃ layer shows (001) orientation onthe side of the rake face and on the side of the flank face.

Since the α-Al₂O₃ layer in each of Comparative Examples 1 to 6 wasformed under the conditions according to the conventional art which arenot in accordance with the method of the present invention, this α-Al₂O₃layer was formed of a crystal texture not exhibiting characteristics asin the present invention (see Table 5).

TABLE 4 Construction of Coating (μm) Type of Base Underlying LayerTiC_(x)N_(y) Intermediate Outermost Layer Total Thickness of Material(TiN layer) Layer Layer α-Al₂O₃ Layer (TiN layer) Coating (μm) Example 1P 1.2 13.0 TiBNO (0.7) A (8.6) 0.9 24.4 Example 2 P 0.5 13.4 TiBNO (0.6)B (7.1) 1.2 22.8 Example 3 P 0.6 16.0 TiBNO (0.9) C (7.1) 0.6 25.2Example 4 P 0.6 8.7 TiBNO (0.6) D (5.4) 0.9 16.2 Example 5 P 0.8 8.6TiCNO (0.7)  E (5.7) 1.0 16.8 Example 6 P 0.7 15.0 TiCNO (0.6)  F (6.6)— 22.9 Example 7 P 0.5 11.2 TiCNO (0.7) G (5.3) 1.1 18.8 Example 8 K 0.88.5 TiBNO (0.7) A (3.9) — 13.9 Example 9 K 1.1 5.4 TiBNO (0.7) B (4.2)1.0 12.4 Example 10 K 0.6 7.3 TiCNO (0.8) C (5.3) 1.3 15.3 Example 11 K0.8 5.5 TiBNO (0.7) D (2.5) 0.5 10.0 Example 12 K 0.4 5.7 TiBNO (0.5)  E(2.3) 1.0 9.9 Example 13 K 0.5 8.1 TiCNO (0.9)  F (4.6) 0.6 14.7 Example14 K 0.7 6.4 TiCNO (0.9) G (4.0) 1.1 13.1 Example 15 K 0.6 6.6 TiCNO(1.1) D (4.6) — 12.9 Comparative P 0.6 14.6 TiCNO (0.7) X (7.5) 1.3 24.7Example 1 Comparative P 0.5 13.7 TiCNO (0.8) Y (7.3) — 22.3 Example 2Comparative P 0.6 13.5 TiCNO (0.6)  Z (7.9) 1.5 24.1 Example 3Comparative K 0.6 7.2 TiCNO (0.8) X (4.6) 1.3 14.5 Example 4 ComparativeK 0.5 6.8 TiCNO (0.8) Y (4.8) — 12.9 Example 5 Comparative K 0.6 6.0TiCNO (0.6)  Z (4.8) 1.4 13.4 Example 6

TABLE 5 Ratio of L_(R3)/L_(R3-29) L_(R3)/L_(R) L_(F3)/L_(F3-29)L_(F3)/L_(F) (L_(R3)/L_(R3-29)) − (001) Orientation (%) (%) (%) (%)(L_(F3)/L_(F3-29)) Rake Face Flank Face Example A 89 17 94 20 −5 57 73 B86 15 92 18 −6 55 62 C 90 18 99 22 −9 60 78 D 83 10 89 13 −6 52 58 E 8511 91 14 −6 54 60 F 84 12 92 18 −8 72 63 G 95 20 99 21 −4 75 80Comparative X 75 3 74 5 1 52 50 Example Y 82 5 81 7 1 42 40 Z 50 1 51 2−1 13 15

<Cutting Test>

Two types of cutting tests below were conducted on the surface-coatedcutting tools obtained above. The cutting test below belongs to thehigh-speed and low-feed cutting process.

<Cutting Test 1>

For the surface-coated cutting tools in Examples and ComparativeExamples shown in Table 6 below, under cutting conditions below, a timeperiod of cutting until an amount of wear of the flank face (Vb) reached0.20 mm was counted and a final form of damage of a cutting edge wasobserved. Table 6 shows results. A longer time period of cuttingindicates better resistance to wear and longer life of the tool.

<Conditions for Cutting>

Work material: Cutting of outer circumference of round rod of SCM435

Peripheral speed: 400 m/min.

Feed rate: 0.1 mm/rev

Cutting depth: 1.0 mm

Coolant: used

TABLE 6 Time Period of Cutting (Minute) Example 1 19 Example 2 20Example 3 23 Example 4 20 Example 5 21 Example 6 22 Example 7 16Comparative 10 Example 1 Comparative 13 Example 2 Comparative 6 Example3

Example 3

As is clear from Table 6, the surface-coated cutting tools in Examplesare better in both of resistance to wear and resistance to chipping andlonger in life of the tool than the surface-coated cutting tools inComparative Examples. Namely, it could be confirmed that mechanicalcharacteristics of the coating in the surface-coated cutting tools inExamples were improved.

<Cutting Test 2>

For the surface-coated cutting tools in Examples and ComparativeExamples shown in Table 7 below, a time period of cutting until anamount of crater wear (Kt) reached 0.20 mm under cutting conditionsbelow was counted. Table 7 shows results. A longer time period ofcutting indicates better resistance to wear and longer life of the tool.

<Conditions for Cutting>

Work material: Cutting of outer circumference of round rod of S55C

Peripheral speed: 300 m/min.

Feed rate: 0.05 mm/rev

Cutting depth: 2.0 mm

Coolant: used

TABLE 7 Time Period of Cutting (Minute) Example 8 21 Example 9 16Example 10 28 Example 11 22 Example 12 26 Example 13 24 Example 14 17Example 15 14 Comparative 9 Example 4 Comparative 12 Example 5Comparative 6 Example 6

As is clear from Table 7, the surface-coated cutting tools in Examplesare better in resistance to breakage and longer in life of the tool thanthe surface-coated cutting tools in Comparative Examples. Namely, itcould be confirmed that mechanical characteristics of the coating in thesurface-coated cutting tools in Examples were improved.

<Confirmation of Effect of Surface Roughness Ra of α-Al₂O₃ Layer>

Surface roughness Ra of the α-Al₂O₃ layer in the surface-coated cuttingtools in Examples 1, 2, and 11 was measured under JIS B 0601 (2001).Table 8 shows results.

Then, surface-coated cutting tools in Examples 1A, 2A, and 11A werefabricated by subjecting the α-Al₂O₃ layer of each surface-coatedcutting tool to aerolap treatment under conditions below. Surfaceroughness Ra of the α-Al₂O₃ layer in the surface-coated cutting tool wasmeasured similarly to the above. Table 8 shows results.

<Conditions for Aerolap Treatment>

Media: elastic rubber media with a diameter of approximately 1 mm, whichcontain diamond grains having an average grain size of 0.1 μm (atrademark: “MultiCone” manufactured by Yamashita Works Co., Ltd.).

Projection pressure: 0.5 bar

Time period of projection: 30 seconds

Wet/dry: dry

For the surface-coated cutting tools in Examples 1, 1A, 2, 2A, 11, and11A, a time period of cutting until an amount of wear of the flank face(Vb) reached 0.20 mm under cutting conditions below was counted. Table 8shows results. A longer time period of cutting indicates stablecapability to discharge chips, with a coefficient of friction betweenchips and the cutting edge of the tool being lower.

<Conditions for Cutting>

Work material: Cutting of outer circumference of round rod of SS400

Peripheral speed: 300 m/min.

Feed rate: 0.1 mm/rev

Cutting depth: 1.0 mm

Coolant: not used

TABLE 8 Surface Roughness Time Period of Ra (μm) Cutting (Minute)Example 1 0.33 27 Example 1A 0.12 40 Example 2 0.30 30 Example 2A 0.1053 Example 11 0.28 35 Example 11A 0.10 60

As is clear from Table 8, it could be confirmed that the surface-coatedcutting tools in Examples 1A, 2A, and 11A including the α-Al₂O₃ layerhaving surface roughness Ra less than 0.2 μm could achieve lowering incoefficient of friction between chips and the cutting edge of the tooland exhibited stable capability to discharge chips, as compared with thesurface-coated cutting tools in Examples 1, 2, and 11 including theα-Al₂O₃ layer having surface roughness Ra equal to or more than 0.2 μm.

<Confirmation of Effect of Compressive Stress Provided to α-Al₂O₃ Layer>

For the surface-coated cutting tools in Examples 1, 2, and 11, it wasconfirmed that there was a point where an absolute value for stress wasmaximal, in a region within 2 μm from the surface side of the coating inthe α-Al₂O₃ layer, and the absolute value of stress at that point wasmeasured. Table 9 shows results (in a field of “stress value”). Stresswas measured with the sin²ψ method using X-rays, and a numeric value inthe field of “stress value” in Table 9 shows an absolute value, withtensile stress being denoted as “tensile” and compressive stress beingdenoted as “compressive”.

Then, surface-coated cutting tools in Examples 1B, 1C, 2B, 2C, and 11Bwere fabricated by subjecting the α-Al₂O₃ layer of each surface-coatedcutting tool to wet blast treatment under conditions below. Then, foreach surface-coated cutting tool, similarly to the above, it wasconfirmed that there was a point where an absolute value of stress wasmaximal, in a region within 2 μm from the surface side of the coating inthe α-Al₂O₃ layer, and the absolute value of stress at that point wasmeasured. Table 9 shows results (in a field of “stress value”).Difference in stress between Example 1B and Example 1C and betweenExample 2B and Example 2C is attributed to a difference in projectionpressure in wet blast treatment.

<Conditions for Wet Blast Treatment>

Media: alumina media (φ50 μm)

Projection pressure: 1 to 2 bars

Time period of projection: 10 seconds

Wet/dry: Wet

A time period of cutting until the tool is broken under conditions forcutting below was counted for the surface-coated cutting tools inExamples 1, 1B, 1C, 2, 2B, 2C, 11, and 11B. Table 9 shows results. Alonger time period of cutting indicates suppression of breakage of thecutting edge of the tool due to mechanical and thermal fatigue whichoccurs during an intermittent cutting process and resultant improvementin reliability of the cutting edge.

<Conditions for Cutting>

Work material: SUS304 (cutting of outer circumference of 60°×3 grooves)

Peripheral speed: 250 m/min.

Feed rate: 0.05 mm/rev

Cutting depth: 1.0 mm

Coolant: not used

TABLE 9 Time Period of Stress Value (GPa) Cutting (Minute) Example 1 0.7(Tensile) 12 Example 1B 0.8 (Compressive) 22 Example 1C 1.0(Compressive) 20 Example 2 0.8 (Tensile) 10 Example 2B 0.6 (Compressive)20 Example 2C 0.2 (Compressive) 14 Example 11 0.6 (Tensile) 15 Example11B 0.8 (Compressive) 27

As is clear from Table 9, it could be confirmed that a point where theabsolute value of stress was maximal was included in the region within 2μm from the surface side of the coating in the α-Al₂O₃ layer, breakageof the cutting edge of the tool due to mechanical and thermal fatiguewhich occurred during an intermittent cutting process was suppressed ina case that stress at that point was compressive stress of whichabsolute value was smaller than 1 GPa as compared with a case thatstress at that point was tensile stress, and consequently reliability ofthe cutting edge improved.

Though the embodiment and the examples of the present invention havebeen described above, combination of features in each embodiment andexample described above as appropriate and various modifications thereofare also originally intended.

It should be understood that the embodiment disclosed herein isillustrative and non-restrictive in every respect. The scope of thepresent invention is defined by the terms of the claims, rather than theembodiment above, and is intended to include any modifications withinthe scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

1 rake face; 2 flank face; 3 cutting edge ridgeline portion; 10 tool; 11base material; 11 a rake face; 11 b flank face; 11 c cutting edgeridgeline portion; and 12 coating.

The invention claimed is:
 1. A surface-coated cutting tool having a rakeface and a flank face, comprising: a base material; and a coating formedon the base material, the coating including an α-Al₂O₃ layer, theα-Al₂O₃ layer containing a plurality of crystal grains of α-Al₂O₃, agrain boundary of the crystal grains including a CSL grain boundary anda general grain boundary, the α-Al₂O₃ layer on a rake face side showing(001) orientation, in the α-Al₂O₃ layer on the rake face side, a lengthL_(R3) of a Σ3 crystal grain boundary in the CSL grain boundaryexceeding 80% of a length L_(R3-29) of a Σ3-29 crystal grain boundaryand being not lower than 10% and not higher than 50% of a total lengthL_(R) of all grain boundaries which is a sum of the length L_(R3-29) anda length L_(RG) of the general grain boundary, the α-Al₂O₃ layer on aflank face side showing (001) orientation, in the α-Al₂O₃ layer on theflank face side, a length L_(F3) of a Σ3 crystal grain boundary in theCSL grain boundary exceeding 80% of a length L_(F3-29) of a Σ3-29crystal grain boundary and being not lower than 10% and not higher than50% of a total length LF of all grain boundaries which is a sum of thelength L_(F3-29) and a length L_(FG) of the general grain boundary, anda ratio L_(R3)/L_(R3-29) of the length L_(R3) to the length L_(R3-29)being lower than a ratio L_(F3)/L_(F3-29) of the length L_(F3) to thelength L_(F3-29), and the (001) orientation being defined as such acondition that a ration of the crystal grains of α-Al₂O₃ of which normaldirection to a (001) planes is within ±20° with respect to a normaldirection to a surface of the α-Al₂O₃ layer is not lower than 50% in theα-Al₂O₃ layer.
 2. The surface-coated cutting tool according to claim 1,wherein the CSL grain boundary is constituted of the Σ3 crystal grainboundary, a Σ7 crystal grain boundary, a Σ11 crystal grain boundary, aΣ17 crystal grain boundary, a Σ19 crystal grain boundary, a Σ21 crystalgrain boundary, a Σ23 crystal grain boundary, and a Σ29 crystal grainboundary, the length L_(R3-29) is a total sum of lengths of the Σ3crystal grain boundary, the Σ7 crystal grain boundary, the Σ11 crystalgrain boundary, the Σ17 crystal grain boundary, the Σ19 crystal grainboundary, the Σ21 crystal grain boundary, the Σ23 crystal grainboundary, and the Σ29 crystal grain boundary in the α-Al₂O₃ layer on therake face side, and the length L_(F3-29) is a total sum of lengths ofthe Σ3 crystal grain boundary, the Σ7 crystal grain boundary, the Σ11crystal grain boundary, the Σ17 crystal grain boundary, the Σ19 crystalgrain boundary, the Σ21 crystal grain boundary, the Σ23 crystal grainboundary, and the Σ29 crystal grain boundary in the α-Al₂O₃ layer on theflank face side.
 3. The surface-coated cutting tool according to claim1, wherein the α-Al₂O₃ layer has a thickness from 2 to 20 μm.
 4. Thesurface-coated cutting tool according to claim 1, wherein the α-Al₂O₃layer has surface roughness Ra less than 0.2 μm.
 5. The surface-coatedcutting tool according to claim 1, wherein the α-Al₂O₃ layer includes apoint where an absolute value for compressive stress is maximal, in aregion within 2 μm from a surface side of the coating, and the absolutevalue for compressive stress at the point is lower than 1 GPa.
 6. Thesurface-coated cutting tool according to claim 1, wherein the coatingincludes a TiC_(x)N_(y) layer between the base material and the α-Al₂O₃layer, and the TiC_(x)N_(y) layer contains TiC_(x)N_(y) satisfyingatomic ratio relation of 0.6≦x/(x+y)≦0.8.
 7. The surface-coated cuttingtool according to claim 1, wherein the length L_(R3) and the lengthL_(F3) of the Σ3 crystal grain boundary are obtained from across-section formed when the α-Al₂O₃ layer is cut along a planeincluding the normal line to a surface of the α-Al₂O₃ layer.